Process for the electrolytic production of alkali metal phosphates



United States Patent 12 Claims. (cl. zoo-90 The present invention isconcerned with a process for the electrolytic production of alkali metalphosphates from phosphoric acid and alkali metal chlorides.

Alkali metal phosphates are produced in most cases, and most simply, byneutralizing phosphoric acid with alkali metal hydroxide or alkali metalcarbonate. Since these two alkali metal compounds are relativelyexpensive, attempts have been made to replace them with alkali metalchlorides.

The reaction between phosphoric acid and the alkali metal chloride is,however, rather diflicult to carry out. The reaction mixture must beheated with evaporation of water and the mixture becoming increasinglymore viscous. Finally, a melt of low' water content is obtained, whichis very viscous in the temperature range up to about 350 C. and fromwhich the resultant hydrogen chloride escapes only with difficulty. Thereaction mixture, therefore must either be heated to a still highertemperature or a long reaction time must be tolerated.

At these high temperatures, it is then particularly difficult to find asuitable material for making the reaction vessel.

Furthermore, the monophosphates which are primarily produced areconverted into condensed phosphates which then may possibly be obtainedin insoluble form.

It has also been proposed to spray suspensions of finely ground alkalimetal chloride in phosphoric acid in a heated tower. Apart from the highinvestment costs for this process, the separation of the resultantproduct from the ot'fgas presents considerable difficulties. Stillfurther, it has already been proposed to prepare alkali metal phosphatesfrom phosphoric acid and alkali metal chlorides by electrolysis. In thisprocess, a solution of H PO and alkali metal chloride in water iselectrolyzed between an anode of graphite and a cathode of nickel. When25 to 50% by weight of the theoretically possible amount of alkali metaldihydrogen phosphate has been formed, the electrolysis is interruptedand the resultant product crystallized, the mother liquor being usedagain for a new batch.

It has now unexpectedly been found that higher yields in theelectrolysis can be obtained by using substantially more concentratedsolutions as the starting material. In the known process, the startingmaterial is an aqueous solultion which contains about 20% by weight H POand about 12% by weight NaCl or about 15% by weight KCl.

ln contradistinction thereto, the starting material used in the presentprocess is phosphoric acid of at least 50%, preferably 5070%, strength,in which the entire required amount of alkali metal chloride issuspended. This fact ensures a high chloride concentration over aprolonged period of time during the electrolysis, the formation ofoxygen at the anode which leads to the destruction of the electrode andcontamination of the chlorine developed, being substantially suppressed.Suspensions containing alkali metal chloride in a concentration of 350to 650 grams per liter have proved advantageous.

The solid phase of alkali metal chloride disappears only when thechloride content of the electrolyte is less than ice about 5% by weight.The electrolysis is interrupted when the chloride content has dropped to2.0 to 2.5% by weight; at that time, about 85 to 88% by weight of thealkali metal chloride introduced has been converted into monoalkalimetal orthophosphate. Below this chloride concentration, the previouslylow oxygen content in the chlorine developed increases suddenly tovalues above 10% by volume. As a result of this, the current yield ofcourse also drops suddenly. From the start of the electrolysis, up tothis point, the oxygen content in the chlorine does not rise above 1% byvolume, it being most of the time substantially below this value.

The electrolysis is carried out preferably at a temperature of 40 to 100C., and preferably 60 to C., so that plastics, such as the relativelyinexpensive polyvinyl chloride can be suitable used as material for thevessel. The temperature is reached by the jouleant heat itself so thatno special heating devices are necessary.

In order to recover the alkali metal phosphate formed, it is merelynecessary to cool the solution with the resultant formation ofcrystallized salt; evaporation is out necessary, in contradistinction tothe known method, where considerably more dilute solutions are used. Theenergy saved for evaporation represents another great advantage of theprocess of the present invention.

Due to the fact that in the present invention the electrolysis leads tohigher yields of to 88% by weight, the separation of the resultantalkali metal phosphate from the rest of the alkali metal chlorides stillpresent in the solution is simpler to effect; i.e., alkali metalphosphates of lower chloride contents can be obtain, as a result ofwhich the possible uses of the phosphates obtained by the electrolyticprocess are increased.

The mother liquor which remains after the crystallization and containsphosphoric acid and alkali metal chloride in addition to alkali metalphosphate is used again in a new batch. The electrolysis batches are soadjusted that there is obtained a P O /Me 0 molecular ratio (Me alkalimetal) of about 1:1. It is not necessary to use an excess of P 0 as ispreferably done in the known processes. An excess of P 0 i.e., ofphosphoric acid, would reduce the chloride concentration, whereby therewould again be the danger of increased oxygen formation. The operationwithout excess acid offers the further advantage that there is noincreased danger of corrosion caused by the electrolysis solution,particularly in the vessels and apparatus serving for the separation ofthe crystals.

Corrosive attack on the apparatus parts by the monoalkali metalphosphate solution which still has an acid reaction can also be avoidedby neutralizing the solutions coming from the electrolysis, beforeseparation of the crystals, with alkali metal hydroxides and/or alkalimetal carbonates to the stage of dialkali metal phosphate. The furthertreatment can then take place in vessels of ordinary iron. This featureotters the further advantage that greater yields, due to the lessersolubility of the dialkali metal phosphates, are obtained.

The electrolysis can be carried out in the known cells, in whichconnection the material of which the vessel is made must be resistant toacid and chlorine. If the chlorine and the hydrogen are to be collectedseparately, a cell with diaphragms or a partition must be employed. Aselectrodes, there can be used the graphite electrodes customarilyemployed in alkali metal chloride electrolysis. The current densityshould be between about 5 and 50 amperes per square dccimeter. Theelectrolyzcd solutions can be worked up in accordance with the customaryknown methods.

Another advantage of the present method is that alkali metal chloridesof lesser purity can also be used, for incorresponding to a currentyield of 86%.

stance products which contain sulfate ions or alkaline earth metals.Such starting substances cannot be used for instance in the skull metalchloride electrolysis with mercury cathode without costly priorpurification, since they lead to contamination of the mercury and thusto diflicultics in the carrying out of the process. In this electrolysiscarried out in accordance with the process of the present invention, theimpurities do not at all affect the course of the process, but merelythepurity of the resultant phosphate, since they are deposited also uponthe crystallization. If for one specific reason or other, noparticularly high demands are madeas to the purity of the product to beproduced, such start ng materials can be readily employed.

The 'following examples illustrate the invention:

Example I In a polyvinyl chloride vessel a suspension of 750 grams NaClin 2500 grams of 50.3% H PO was electrolyzed, .while stirring, with acurrent of 25 amperes (current density about 10 aJdm?) using twographite electrodes of a size of 175 x 140 mm., until the chloridecontent of the solution had dropped to 2.3%. The tempernture of theelectrolyte was about 70 C. 637 grams or 85 of the NaCl was converted tomonosodinrn phosphate. The electrolysis lasted for 14 hours, i.e., 350ampere hours were consumed, corresponding to a current yield of 83.5%.The voltage was 3.8 volts. Half of the solution obtained wascrystallized by cooling, 425 grams of NaH PO having a chloride contentof 1.1% being obtained. The other half was adjusted to a pH of 9 withsodium hydroxide solution. Upon cooling it, 739 grams of Na HPOcontaining 1.3% chloride were obtained.

Example 2 The apparatus was the same as that used in Example 1. Asuspension of 970 grams of KCl in 2500 grams of 51% phosphoric acid was'electrolyzed with a current of 25 amperes at about 65 C., until achloride content of 2.0% was reached. 834 grams KCl (86%) were convertedto KH PO 350 ampere hours were consumed, The voltage was 3.7 volts. 1370grams KH3P04 containing 0.2%, chloride crystallized from the solutionwhile about 150 grams KH3P04 remained in the mother liquor.

Example 3 In a bell-type cell, which permitted the collection of thechlorine formed, a suspension of 244 grams NaCl in 750 grams H POI,(53.3%) was elcctrolyzed with a current of amperes (current densityabout 12 amperes per square decimeter) until the chloride content haddropped to 2.5%. 208 grams of NaCl corresponding to 85% were converted.The time of electrolysis was 22 hours, corresponding to 110 ampere hours(current yield 87% The oxygen content of the chlorine was immeasurablysmall at the start of the electrolysis; after 11 hours, it had risen to0.5% and at the end of the electrolysis, was 1.0%. 300 grams of NaH POwith a chloride content of 0.4% were crystallized from the solutionwhile 128 grams remained in the mother liquor.

We claim:

1. In a process for the electrolytic production of alkali metalphosphates from phosphoric acid and alkali metal chlorides, theimprovement which comprises electrolyzing a suspension of an alkalimetal chloride in aqueous phosphoric acid having an initia'concentration of at least 50% by weight, crystallizing the resultingphosphate, and separating mother liquid and phosphate from .one another.

2. The process of claim 1, wherein the phosphoric acid has an initialconcentration within the range of 50 to by weight.

3. The process of claim 1, wherein the initial concentration of alkalimetal chloride in the electrolyte is within the range of 300 to 650grams per liter of electrolyte. I

4. The process of claim 1, wherein the electrolysis is interrupted whenthe chloride content of the electrolyte has dropped to a value withinthe range of 2.0 to 2.5% by weight.

5. The process of claim 1, wherein the electrulysis is carried out attemperatures with the range of 40 to 100 C.

6. The process of claim 1, wherein the electrolysis is carried out attemperatures within the range ot 6 0 to C.

7. The process of claim 1, wherein the ratio of P 0 :Me 0, in which Meis an alkali metal, is approximately 1:1 in the electrolyte.

8. The process of claim 7, wherein Me represents at least one substanceselected item the group consisting of sodium and potassium.

9. The process of claim 1, wherein current having a density within therange of about Ste 50 amperes per square decimeter is used for carryingout the electrolysis.

10. The process of claim 1, wherein the solution obtained aftercompletion of the reaction is neutralized with at least one substanceselected from the group consisting of alkali metal hydroxides and alkalimetalcarbonates, the solution being neutralized to the stage of dialkalimetal phosphates. Y

11. The process of claim 1, wherein the mother liquor obtained onprocessing the phosphates is usedagain for anew batch. I

. 12. The process of claim 11, wherein the phosphates are processed bycrystallization by cooling.

References Cited by the Examiner UNITED STATES PATENTS 572,512 12/1s96Albert 204- FOREIGN PATENTS 854,145 1 l/ 1960 Great Britain.

IOHN H. MACK, Primary Examiner. MURRAY TILLMAN, Examiner. L.G. WISE,Assistant Examiner.

1. IN A PROCESS FOR THE ELECTROLYTIC PRODUCTION OF ALKALI METALPHOPHATES FROM PHOSPHORIC ACID AND ALKALI METAL CHLORIDES, THEIMPROVEMENT WHICH COMPRISES ELECTROLYZING SUSPENSION OF AN ALKALI METALCHLORIDE IN AQUEOUS PHOSPHORIC ACID HAVING AN INITIAL CONCENTRATION OFAT LEAST 50% BY WEIGHT, CRYSTALLIZING THE RESULTING PHOSPHATE, ANDSEPARATING MOTHER LIQUID AND PHOSPHATE FROM ONE ANOTHER.